Hitherto, an attempt has been made for controlling state of flow of a molten steel in a mold by applying a static magnetic field for the purpose of reducing any local deviation or uneven distribution of flow of the molten steel which tends to occur when the molten steel is poured into the mold. In such a method relying upon application of a static magnetic field, it is necessary that a path is formed to enable free flowing of an induction current which is generated as a result of interference between the static magnetic field and the flowing molten steel, corresponding to the outer product U .times.B of the flowing velocity U of the molten steel and the intensity B of the magnetic field. For instance, in a method shown in FIG. 6 in which a static magnetic field is applied substantially uniformly, an induction current 6 (see FIG. 7) tends to be generated due to interaction between the static magnetic field and the flow of molten steel. The induction current, however, cannot flow unless a path for circulation of such a current is provided. Consequently, it is necessary to form a bypass current which passes through the region near the wall where the magnetic field intensity is low. In order to obtain the bypass current, it is necessary to use an electromotive force large enough to produce such a current.
FIG. 8 illustrates the distribution of the electric potential .phi. which provides the electromotive force for the production of the bypass current. The bypass current (J.sub.1 =-.sigma.grad .phi.) tends to flow from a region where the potential .phi. is high to the region where the potential .phi. is low. The actual current J is the sum of the induction current J.sub.2 (.sigma.U.times.B) and the current J.sub.1 produced by the electromotive force. Thus, the actual current J is expressed as J=J.sub.1 +J.sub.2 =.sigma.(U.times.B -grad .phi.). In consequence, although the bypass current generated by the electromotive force flows in the region near the wall where the magnetic field intensity is low, a potential gradation (grad .phi.) which serves to suppress the induction current J.sub.2 is formed in the region around the discharge flow of the molten steel, so that the actual current J is reduced in such a region. As a consequence, a reduction is caused in the efficiency of the electromagnetic brake (Lorenz force corresponding to the outer product J.times.B of the current J and the magnetic field intensity B). This reduction is generally 50% or greater. In order to obtain the desired electromagnetic force, therefore, it is necessary to apply a larger magnetic force.
In the field of single-crystal growth process in which a single crystal is made to grow and be lifted in accordance with a Czochralski process, it has been proposed to brake a natural convection generated in a melt, as well as forced convection caused by rotation of the crystal or of a crucible, by applying a cusp field as shown in FIG. 9. This art is shown in JP-A-58-217493 and JP-A-61-222984. In contrast to the discharge flow of molten steel in a continuous casting mold, the flow of the melt in the single-crystal growth process occurs in the regions near the walls of the container which has an axisymmetrical configuration with respect to the axis. This cusp field is generated radially and axisymmetrically, by placing upper and lower electromagnets which oppose each other with the same poles, namely with reverse polarity, so as to surround the single-crystal lifting furnace. It is reported that the cusp field provides a high braking efficiency because it acts perpendicularly to the flow of the melt in the region near the wall so as to enable the induction current to flow circumferentially.
The behavior of the melt in the single-crystal lifting process in which convection is caused by heat from the wall and shear stress generated in the boundary between the melt and the wall is entirely different from the behavior of the melt in the continuous casting of steel in which the melt is jetted and supplied from a immersion nozzle into a mold. Therefore, the manner of application of a magnetic field in the single-crystal lifting process cannot give any hint to the manner of application of a magnetic field to the melt in continuous casting process.